The present application relates to the formation of acrylic fibres by wet spinning polymeric solutions in a nitric acid coagulation bath and simultaneously recovering from the coagulating bath the nitrates produced during the wet spinning.
It is known that nitric acid is a good solvent for polyacrylonitrile and copolymers of acrylonitrile. Aqueous solutions of between 65 to 72 percent ic nitric acid dissolve acrylic polymers at temperatures between zero and 20.degree. C. The solutions thus obtained are spun into a more diluted nitric acid solution,, e.g. 40% by weight. Acrylonitrile polymers cannot be spun into plain water or a less dilute nitric acid solution because the fibres thus produced would be non-homogeneous, weak and could not be easily stretched. Moreover, dilute nitric acid solutions would be disadvantageous from an economical standpoint with respect to the recovery of the acid. Therefore, the concentration of nitric acid in the coagulating bath is usually kept at about 40% and the acid is recovered by distillation and reused thereafter. The wash is carried out in counterflow, in the usual way, but it is impossible to recover all nitric acid because the volume of the dilute wash water is too large. Wash water is generally neutralized and drained off to sewage. This is a very serious problem with respect to current water polution restrictions.
The recovery of nitric acid in the form of nitrates by neutralizing the coagulating bath containing 40% to 45% nitric acid, is not economical either, because of the large quantities of water to be evaporated. To recover nitric acid from large quantities of dilute waste water is practically impossible. On the other hand, the market for nitrates in the fertilizer industry is broad so that their recovery at reasonable costs would be desirable.
It has been established that in wet spinning processes as well as in such analogous manufacturing processes as in the formation of foils, tubings and similar conversion of acrylic polymers and copolymers into solid form, the coagulation is, in principal always heterogeneous. At first, a more or less thin homogeneous coagulated membrane ("skin") is formed. The thickness of this skin depends upon the kind of polymer, the solvent, the temperature and other factors such as the bath length, the velocity of the withdrawal of the formed polymer, as well as the agitation of the bath and other physical factors. After the formation of the skin, the molecules of the solvent in the polymeric solution penetrate from the interior of the fibre or the like through the outer membranous skin into the spinning coagulating bath. The molecules of the coagulating agent (usually water) diffuse in opposite directions into the fibre. Where acrylonitrile polymers and copolymers are dissolved in nitric acid solutions, the water molecules penetrate through the membranous skin at a considerably higher rate than the nitric acid molecules and ions diffuse outwardly. Thereby an undesirable osmotic pressure is created, leading to the non-homogeneous formation of the fibre. Water penetration through the surface membrane dilutes the nitric acid solution and coagulates the polymer, forming in addition locally thin membranes which burst by the osmotic pressure of the water. This is repeated until the coagulation is finished, leaving the interior of the fibre full of microscopic voids filled with water. Such fibres are weak, brittle and cannot be stretched.
Nevertheless homogeneous, stretchable fibres can also be obtained, but only if the thickness of the surface membrane is at least half of that of the fibre diameter.
It has been further established that such thick membranes are obtained only if the diffusion rate of the coagulating agent is not substantially greater than that of the solvent. Up to now, this problem was solved in various ways considered broadly as methods of decreasing the coagulation rate of the spinning solution. The rate of coagulation was decreased either by decreasing the temperature of the spinning bath below plus 2.degree. C or by maintaining a rather high concentration of solvent in the bath. Still another method used was a two step coagulation procedure. The intermediate fibre produced was a rubbery plasticized fibre containing a certain amount of solvent which was then washed out from the elastically stretched polymer. The first mentioned method was first obtained by the Dow Chemical Company in spinning zinc chloride solutions of polyacrylonitrile, but the same method can be used also for other solvents. In all of the above mentioned methods the principle is basically the same: to decrease the activity of water toward the polymer solution.
It had also been proposed to use copolymers of acrylonitrile with hydrophilic comonomers such as acrylamide so that water enters into the fibre being formed and acts as a plasticizer. Furthermore it was also known that even non solvents for acrylic polymers, such as various salts, can reduce the coagulation activity of the bath so as to obtain translucent strong gels. On the other hand it was known that strong bases rapidly hydrolyse the nitrile group into a carboxylic one and moreover, cause rapid discoloration. In the case where nitric acid is used as the solvent for acrylic polymers, bases could not be added to the coagulating liquid since the instantaneous neutralization would cause an extreme increase the coagulation rate and thus cause heterogeneity of the fibre which would be full of microscopic voids.
The present invention is on the other hand based upon the activity of the coagulant, e.g. water, being decreased so that the diffusion of water through the surface membrane into the coagulated stream (fibre) of the polymer solution is not substantially higher than that of the solvent diffusing outwards. In the known processes the activity of water remains high enough to form a gel-like fibre containing more water than the same polymer in a true equilibrium with water. The surplus of water is present as a "false plasticizer", not really solvating the polymer but forming a non-equilibrium mixture in which water fills up rather uniformly the interstices between the chains. Such gel-like fibres are stretchable but their strength is not too high because not only is the concentration of the solvent decreased but so is the concentration of the polymer. When stretched and dried, they cannot be converted again into the original gel state. Even hydrophilic copolymers do not regain their original water content if coagulated in the usual way because said surplus of water, forming the false plasticizer can be introduced only if the coagulation occurs in an excess of water, increasing by its osmotic pressure the volume of the coagulated fibre. The water molecules are pushed between the chains prior to the production of intermolecular bonds.
On the other hand, it is possible to decrease the activity of the water in the coagulating bath to such an extent that is diffusion rate is considerably lower than that of the solvent. This can be obtained when a rather concentrated electrolyte solution, preferably a nitrate solution, is use as a coagulating bath. In a saturated solution the activity of water sinks to zero and therefore the solution should be maintained below the saturation point. In this condition the solvent (nitric acid) penetrates more rapidly outward through the surface membrane than the water would inwardly. As a result the concentration of the polymer in the stream increases during the coagulation and the coagulated fibre contains no excessive water, i.e., no false plasticizer. The fibre is thus stronger and the spinning is not so often interrupted by torn filaments. In spite of the decreased activity of the coagulant, the total rate of coagulaton is maintained high by adding a neutralizing agent to the coagulating liquid during the wet spinning. Small hydrogen ions diffuse very rapidly from the fibre followed by their counter-ions, the NO.sub.3.
It is therefore the object of the present invention to provide a new method for manufacturing acrylic fibres and filaments by the wet spinning of nitric acid solutions of acrylic polymers and copolymers into an aqueous coagulating bath wherein a comparatively high concentration of nitrates is constantly maintained and wherein the nitrates are simultaneously recovered during the process.
It is a further object of the present invention to provide a new method for manufacturing acrylic fibres and filaments and the recovery of nitrates overcoming the disadvantages and defects of the prior art methods.